When the keyboard focus is in an editable cell,
there doesn't appear to be a way to start editing
the contents of the cell. (in contrast, it is
possible to do this in the property sheet).
To reproduce: display the Local Variables view,
and move the keyboard focus to the Value column.
How to edit this?

Fixed in property panel rewrite branch - you can now press space to
edit either the current cell (if not read only) or the nearest
read-write cell to the left of the current cell (excluding the node name).